1. Field of the Invention
The present invention relates to a surface treatment of an article and in particular to surface treatment of a metallic article to prevent generation of microcontamination by the article.
2. Description of the Prior Art
Close tolerance, precision equipment is vulnerable to dust and other microcontaminants. In a hard disk drive, for example, the flying head may be separated from the surface of the spinning disc of recording media by a distance as small as about 0.25 micrometers. A particle of dust interrupting the small separation space can cause a "head crash" and result in damage to both equipment and magnetically recorded data. Therefore, durable, dependable close tolerance, precision equipment requires internal components which do not generate microcontaminants.
Typically, critical metallic components for such precision equipment have been machined from wrought aluminum or magnesium then surface finished by passivation. A less expensive method of providing dimension-critical metallic components is die-casting. Die-cast components, however, have microporous surfaces and tend to carry traces of lubricants, oils and mold release chemicals in the micropores and on the surface. These chemicals also interfere with the passivation process, and result in passivation layers which are subject to peeling. Peeling of the passivation layer, in turn, exposes the revealed metal to corrosion. Peeling passivation layers and corroding metal are both sources of microcontaminants which are detrimental to durability and dependability in precision equipment such as disc drives.
Peeling also exposes additional surface microposres, thereby freeing any contaminants trapped therein. If the component outgases (i.e. releases vapors from hydrocarbon fluids carried in the surface or in micropores), then a film-like coating of hydrocarbon contaminants can deposit upon the head and disk. The hydrocarbon contaminant coating adversely affects the tribological properties of the hard disk drive increasing stiction and wear. In hard disk drives, many components made of aluminum and magnesium alloy systems are used. Many of these critical components are machined from wrought aluminum alloys such as 6061-T6 and 7075-T6. Alternatively, some components are made from sheet metal aluminum alloys such as 5052 and 5086 and die-cast aluminum alloys such as 360 and 380. Other components, such as the head arm carriage are made of magnesium die-cast AZ-91C alloy. There are also components made of sand-cast, investment-cast and powder-metallurgy alloys.
Die-cast components could be more widely used in precision equipment if the problem of microcontamination could be solved.
The prior art includes surface treatment processes which are not specifically aimed at preventing microcontamination. For example, Lamont U.S. Pat. No. 4,756,810 discloses depositing an aluminum alloy onto a substrate (such as a semiconductor wafer), followed by redistributing the deposit by bombarding with ions. Church U.S. Pat. No. 4,652,954 discloses depositing, by vacuum evaporation or sputtering, a thin film of NiFe upon a ceramic substrate. After electroplating additional metal upon the film, another over layer of aluminum oxide is sputter deposited. Katayose U.S. Pat. No. 3,099,609 discloses a process of removing the oxidized surface film from aluminum alloy material; cleaning the surface with a mixed solution of HCl and HF; anodizing with a solution of chromic acid, sulfuric acid or phosphoric acid; and electroplating chromium onto the surface.